This invention relates to an erasable optical data storage medium and a method for optically recording, storing, and erasing data on such a medium, and more particularly to such a method and medium where an optically detectable deformation is created by a portion of the medium undergoing viscoelastic shear deformation.
Within the past decade there has been tremendous interest and developmental work in the area of optical data recording and storage techniques, primarily because of the significant advantages such technology offers over magnetic data technology including: speed of recording, reading, and erasing; storage capacity; and archival life. One of the reasons optical data technology has not yet supplanted magnetic data techniques is the lack of an effective and economical optical data storage medium which is also erasable.
One optical data storage technique employs a laser beam to burn away or ablate a pit or crater in the surface of a data storage medium such as a disc, thereby creating an optically detectable deformation on the surface of the medium. Representative patents disclosing this ablative technique are Howe U.S. Pat. No. 4,336,545, Bell U.S. Pat. No. 4,285,056 and Orukawa et al. Japanese Patent Application No. 58-62096.
Another technique uses a laser beam to create a bubble or vesicle within the medium by heating a portion of the medium until it gasifies and changes state. Representative patents disclosing this vesicular technique are Cornet U.S. Pat. Nos. 4,577,291, 4,371,954, 4,360,895, 4,405,994, 4,404,656, 4,398,203, Maffit et al. U.S. Pat. No. 4,430,659 and Bell U.S. Pat. No. 4,300,227.
In the ablative and vesicular techniques the medium actually undergoes a change of physical state because the laser beam is used to heat a portion of the medium until it evaporates, a portion of the medium changing to a gas. One of the major problems associated with these techniques is that such a change of state makes it very difficult, if not impossible, to reverse the process and erase the data recorded in the medium.
Accordingly, techniques which create erasable optically detectable deformations in nonvesicular and nonablative fashions are of particular interest. Willis U.S. Pat. No. 4,264,986 and Cornet U.S. Pat. No. 4,371,954 disclose nonablative and nonvesicular techniques, disclosing reversible, hence erasable, changes in the structure of the medium, Willis changing the density of the affected medium, and Cornet '954 changing a portion of the medium between different crystallographic phases.
A pending patent application filed by applicant's assignee also discloses an erasable, nonvesicular, nonablative technique for writing, reading and erasing data. Pursuant to MPEP 608.01(p), commonly owned U.S. patent application Ser. No. 835,960, now U.S. Pat. No. 4,719,615, is incorporated herein by reference, with particular attention drawn to the description of the prior art contained therein and to the system described therein for recording, reading and erasing data.
The technique of the commonly owned prior application employs a dual recording layer having an underlying expansion layer covered by a retention layer. The focused laser beam is used to heat the expansion layer causing localized expansion. Concurrently the retention layer is also heated above its glass transition temperature, either by contact with the heated expansion layer, or directly by a laser beam. The heated expansion layer causes the now pliable retention layer to be strained elastically while above its glass transition temperature. When the medium is allowed to cool, the retention layer falls below its glass transition temperature while the expansion layer is still in an expanded state, the retention layer retaining a portion of the resulting deformation while the expansion layer, which is bonded to the retention layer, is held in tension, in partially expanded condition by the now glassy retention layer.
The medium is erased by using a second laser beam to heat the retention layer above its glass transition temperature and allow the tensioned expansion layer to pull the retention layer flat, thereby erasing the optically detectable deformation from the medium.